Fixed UMS.getDevicePolicyManagerInternal() usage.

It can be null (on devices that doesn't define the device_admin
feature)

Test: atest FrameworksMockingServicesTests --test-filter=".*UserManagerServiceMockedTest.testGetApplicationRestrictionsForUser.*"
Fixes: 428867650
Flag: EXEMPT (trivial bug fix)

Change-Id: I8f9be08f2d7ab9e6d68e2345fad5d169e89dbf35